Screen

Available via ports, screen allows you to create a session that you can detach from and attach to irrespective of whether or not your current connection to the machine remains active; e.g. if you're ssh'ed into a machine, start screen, name a session, start up a long process and then detach screen. Logout and then either go to a new computer or start a new ssh session and you can re-attach to your screen session and still find your process running.

Compare to at and nohup.